Murine cytomegalovirus infection is being studied as a convenient model of human infection and disease. Present studies are designed to evaluate the role of macrophages in primary infection, the influence of host genotype on susceptibility, and patterns of age-dependent susceptibility to infection in these contexts. In addition, virulence and attenuation phenomena are under investigation, especially with regard to the contribution of the input viral genome to subsequent productive or abortive infection. Finally, patterns of antiviral drug sensitivity are being studied both with murine and human strains of cytomegaloviruses.